Revealing the Electrocatalytic Self-Assembly Route from Building Blocks into Giant Mo-Blue Clusters.

The assembly of single-core molybdate into hundreds of cores of giant molybdenum blue (Mo-blue) clusters has remained a long-standing unresolved scientific puzzle. To reveal this fascinating self-assembly behavior, we demonstrate an aqueous flowing in-operando Raman characterization system to capture the building blocks' evolution from the "black box" reaction process. We successfully visualized the sequential transformation of Na 2 MoO 4 into Mo 7 O 24 6- ({Mo 7 }), high nuclear Mo 36 O 112 8- ({Mo 36 }) cluster, and finally polymerization product of [H 6 K 2 Mo 3 O 12 (SO 4 )] n ({Mo 3 (SO 4 )} n ) during the H 2 SO 4 acidification. Notably, the facile conversion of {Mo 3 (SO 4 )} n back to the {Mo 36 } cluster by simple dilution is also discovered. Furthermore, we identified {Mo 36 } and {Mo 3 (SO 4 )} n as exclusive precursors responsible for driving the electrochemical self-assembly of {Mo 154 } and {Mo 102 }, respectively. The study also unravels a pivotal intermediate, the pentagonal reduced state fragment [H 18 Mo VI 4 Mo V O 24 ] - , originating from {Mo 36 }, which catalyzes the autocatalytic self-assembly of {Mo 154 } with electron and proton injection during electrochemical processes. Concurrently, {Mo 3 (SO 4 )} n serves as the indispensable precursor for {Mo 102 } formation, generating sulfation pentagon building blocks of [H 2 Na 2 O 2 (H 4 Mo V Mo VI 4 O 16 SO 4 ) 4 ] 4- that facilitate the consecutive assembly of giant {Mo 102 } sphere clusters. As a result, a complete elucidation of the assembly pathway of giant Mo-blue clusters derived from single-core molybdate was obtained, and H + /e - redox couple is revealed to play a critical role in catalyzing the deassembly of the precursor, leading to the formation of thermodynamically stable intermediates essential for further self-assembly of reduced state giant clusters.